1. Introduction
This invention relates to a metal depositing composition and more particularly, to a non-fuming electroless copper plating solution.
2. Description of the Prior Art
Electroless copper deposition refers to the chemical plating of copper over a clean, catalytically active surface, by chemical reduction in the absence of an external electric current. Such processes, and compositions useful therefor, are known and are in substantial commercial use. They are disclosed in a number of prior art patents, for example, U.S. Pat. Nos. 3,663,242; 3,728,137; 3,846,138; and 4,229,218, all incorporated herein by reference.
Known electroless copper deposition solutions generally comprise four ingredients dissolved in water. They are (1) a source of cupric ions, usually a copper salt such as copper sulphate, (2) a reducing agent such as formaldehyde, or preferably a formaldehyde precursor such as paraformaldehyde, (3) hydroxide, generally an alkali metal hydroxide and usually sodium hydroxide, sufficient to provide the required alkalinity necessary for said compositions to be effective, and (4) a complexing agent for copper sufficient to prevent its precipitation in alkaline solution. A large number of complexing agents are known and described in the aforesaid cited patents and elsewhere.
Known electroless copper plating solutions of the above type frequently provide a plate which, if mechanically dense and strong, is brittle such that it can withstand limited bending or thermal stress without fracture. This is not a substantial disadvantage where the electroless plate is of the order of millionths of an inch in thickness and is overplated with ductile electrolytic copper. However, where the entire desired thickness is provided by electroless plating, i.e., typically one to three mils in electrical fabrication such as in the manufacture of printed circuit boards by additive techniques, limited ductility is a serious limitation.
One means of improving the bending or tensile characteristics of an electroless copper plate is described in U.S. Pat. No. 3,213,430 which discloses the addition to the copper plating solution of a water soluble compound of cyanide, vanadium, molybdenum, niobium, tungsten, rhenium, arsenic, antimony, bismuth, rare earths of the actinium series and rare earths of the lanthanum series. Certain members of the above groups, especially the vanadium compounds, provide improved bending characteristics. The reason for this is not fully understood, but it is stated in the patent that the agents act on the catalytic surface so as to prevent formation and release of hydrogen gas, thereby inhibiting the inclusion of hydrogen in the deposit as it forms. It has been found that where a complexing agent or a bath formulation is used permitting rapid deposition of copper with rapid evolution of hydrogen gas at the surface, the improved ductility or bending characteristics are frequently sacrificed or lost.
An additional method for improving the bending or tensile characteristics of a copper plate is disclosed in U.S. Pat. No. 3,728,137 noted above. In this patent, the copper solution is characterized by the addition of a formaldehyde addition agent to the solution which is believed to be responsible for forming an unstable addition product with formaldehyde. Because of the formation of the addition product with formaldehyde, it is theorized by the patentee that formaldehyde is slowly released into solution thereby reducing the evolution of hydrogen gas at the surface of the part being plated resulting in copper deposits having improved bending or tensile properties.
Though the improvement is obtained as noted in said patent, some free formaldehyde is still found in the bath at all times as a consequence of a breakdown of the addition product. This free formaldehyde is released from solution as a vapor which is dangerous to health and must be removed from the atmosphere, though the release of this formaldehyde to the atmosphere is decreased from that normally encountered absent the addition agent.